A variety of methods have been used to detect flaws in metallic structural materials. Where surfaces are exposed, visual inspection is employed, sometimes with the aid of a fluid which makes minute cracks readily visible in the presence of ultraviolet light.
Other methods of nondestructive evaluation (NDE) employ ultrasonic waves, light scattering, eddy currents and x-rays. All of these methods involve the possible detection of some path deviation of an imposed physical wave phenomenon, either electromagnetic or mechanical in nature. Some of these are portable for use in the field, but just about all of the methods require intimate proximity to the element under test, e.g., ultrasonic NDE techniques require physical contact to the element under test. Other techniques, such as thermal wave imaging and eddy current excitation are effective only within a small depth of the surface. Most techniques are not effective at determining defects well within the interior of a structure. While x-ray analysis is an exception to this, it is not practical to place the most sensitive x-ray equipment near a test element in the field. Also, because fatigue originates in an atomic level phase slip process, it is difficult for any existing technique to detect fatigue in its earliest stages.
This invention makes use of (1) my earlier discovery of a phenomenon unique to iron-based alloys and (2) the extreme sensitivity of SQUID magnetometry to minute changes in magnetic field. It employs also the principle of phase sensitive detection so that small changes in a magnetic field surrounding an iron-containing structure can be related to atomic reorientation within that structure without being masked by unrelated environmental magnetic noise of a larger magnitude.
Phase sensitive detection requires that a periodic mechanical stimulus be applied to the test structure. The output circuit of the SQUID magnetometer is operated at the same frequency as that of the mechanical stimulus, and its phase is adjusted to produce a maximum response. If the phase of the magnetic response is the same as that of the mechanical stimulus (exclusive of fixed internal electronic phase shifts), then this indicates that the magnetic field due to stress within the structure is increasing when the stress is increased by an incremental amount. This is the situation that occurs when the total stress within elements of the structure is less than about 60% of that which produces plasticity, i.e., less than 60% of the elastic limit stress. If the magnetic response is 180.degree. out of phase with the periodic mechanical stimulus, then it is known that the threshold level for atomic phase slip and subsequent fatigue has been exceeded.